Existing clothes dryers mainly include air-vented clothes dryers, condenser clothes dryers, and heat pump clothes dryers. Compared with the air-vented clothes dryers and the condenser clothes dryers, the heat pump clothes dryers have advantages of energy conservation, environment protection and etc. A heat pump system of the heat pump clothes dryer includes a compressor, a condenser, a throttling element and an evaporator. Air for drying materials is driven by a fan to circulate. Firstly the air is heated by a high temperature condenser and enters into a roller to absorb moisture in the materials, thus the air has a high humidity. Then, the high-humidity air is cooled and dehumidified by a low temperature evaporator, and the condensate water formed in the above process is discharged through a drain system. The low-temperature and low-humidity air from the evaporator enters into the condenser to be heated, to become high-temperature and low-humidity air again, and then the high-temperature and low-humidity air enters into the roller to absorb moisture. The above circulation is repeated until the humidity of the material in the roller meets a requirement. In the drying process, with the discharging of the condensate water, the temperature of the materials becomes increasingly higher, and the moisture load in the materials becomes increasingly lower. In this case, if energy provided by a refrigerant system only changes slightly, the temperature of the air, which enters into the roller after being heated by the condenser, would become increasingly higher. Although the moisture absorption capability of the air increases as the air temperature in the roller increases, the materials may be broken by the high-temperature air. And, since the temperature of the air passing through the evaporator becomes increasingly higher, the evaporating pressure and the evaporating temperature becomes increasingly higher, and the suction pressure and the suction temperature of the compressor may also become increasingly higher, thus the discharge pressure and the discharge temperature of the compressor are increased. Therefore, for the heat pump clothes dryer, it is a main problem to control the temperature of the inlet air of the roller and the discharge temperature of the compressor in the drying process, and meanwhile, save energy as much as possible. Currently, some manufacturers address the above problem by providing a subcooler which is not involved in the air circulation system. The subcooler dissipates heat into the atmosphere for controlling the temperature of the inlet air of the roller, that is, to control the heat entering into the air circulation system. However, the heat dissipated into the atmosphere from the subcooler is wasted, causing a low energy utilization efficiency.